Weld Joint

Joint with zero primitives

Library

Joints

Description

This block represents a joint with zero degrees of freedom.
It contains no joint primitives. Base and follower frames, each connected
to a separate rigid body, are coincident for all time. The block dialog
box provides sensing options for constraint and total forces and torques.

Joint Degrees of Freedom

Dialog Box and Parameters

Composite Force/Torque Sensing

Select the composite, or joint-wide, forces and torques to sense.
These are forces and torques that act not at individual joint primitives
but at the whole joint. Options include constraint and total forces
and torques.

During simulation, the block computes the selected composite
forces and torques acting between the base and follower port frames.
It outputs these variables using physical signal output ports. Check
the port labels to identify the output variables at different ports.

Direction

Forces and torques acting at joints do so in pairs. Newton's
third law of motion requires that every action be accompanied by an
equal and opposite reaction. If the base frame of a joint exerts a
force or torque on the follower frame, then the follower frame must
exert an equal and opposite force or torque on the base frame.

Select whether to sense the composite forces and torques exerted
by the base frame on the follower frame or vice versa. The force and
torque vector components are positive if they point along the positive
X, Y, and Z axes of the selected resolution frame.

Resolution Frame

You can resolve a vector quantity into Cartesian components
in different frames. If the resolution frames have different orientations,
then the measured components are themselves different—even
though the vector quantity remains the same.

Select the frame in which to resolve the sensed force and torque
variables. Possible resolution frames include Base and Follower.
The block outputs the Cartesian components of the sensed force and
torque vectors as observed in this frame.

Constraint Force

Joint blocks with fewer than three translational degrees of
freedom forbid motion along one or more axes. For example, the Gimbal
Joint block forbids translation along all axes. To prevent translation
along an axis, a joint block applies a constraint force between its
base and follower port frames. Constraint forces are orthogonal to
joint translation axes and therefore do no work.

Select the check box to compute and output the 3-D constraint
force vector [fcx, fcy, fcz]
acting at the joint. Only constraint force components that are orthogonal
to the joint translational degrees of freedom have nonzero values.
Selecting this option causes the block to expose physical signal port
fc.

Constraint Torque

Joint blocks with fewer than three rotational degrees of freedom
forbid motion about one or more axes. For example, the Cartesian Joint
block forbids rotation about all axes. To prevent rotation about an
axis, a joint block applies a constraint torque between its base and
follower port frames. Constraint torques are orthogonal to joint rotation
axes and therefore do no work.

Select the check box to compute and output the 3-D constraint
torque vector [tcx, tcy, tcz]
acting at the joint. Only constraint torque components that are orthogonal
to the joint rotational degrees of freedom have nonzero values. Selecting
this option causes the block to expose physical signal port tc.

Total Force

A joint block generally applies various forces between its port
frames:

The net sum of the different force components equals
the total force acting between the joint port frames. Select the check
box to compute and output the 3-D total force vector [ftx, fty, ftz].
Selecting this option causes the block to expose physical signal port
ft.

Total Torque

A joint block generally applies various torques between its
port frames:

Constraint torques that forbid motion in directions
orthogonal to the revolute or spherical joint primitive axes.

The net sum of the different torque components equals the total
torque acting at a joint. Select the check box to compute and output
the 3-D total torque vector [ttx, tty, ttz].
Selecting this option causes the block to expose physical signal port
tt.

Ports

This block has two frame ports. It also has optional physical
signal ports for sensing dynamical variables such as forces, torques,
and motion. You expose an optional port by selecting the sensing check
box corresponding to that port.

Frame Ports

B — Base frame

F — Follower frame

Sensing Ports

The following sensing ports provide the composite forces and
torques acting on the joint: